We have fabricated well-defined nanostructures such as
SiO2-coated Ag nanoparticles (NPs) connected with quantum
dots
(QDs) (Ag/SiO2-QDs) so as to control the fluorescence enhancement
induced by localized surface plasmon resonance. Namely, the distance
between Ag NP and QD should be noted as a controllable model to investigate
the fluorescence enhancement effect. Actually, highly monodispersed
Ag NP as a core was first coated with five thicknesses of SiO2 as a shell, and then QDs were specifically adsorbed onto
the surface of the amino-functionalized SiO2-coated Ag
NPs. As a result, the fluorescence intensity increased with the shell
thickness as a result of excitation enhancement. On the other hand,
the fluorescence intensity decreased when the shell thickness became
thinner because of the induced quenching. Therefore, the distance
between Ag NPs and QDs should be optimized to control and enhance
the fluorescence intensity.
Controlling or switching the optical signal from a large collection of molecules with the minimum of photons represents an extremely attractive concept. Promising fundamental and practical applications may be derived from such a photon-saving principle. With this aim in mind, we have prepared fluorescent photochromic organic nanoparticles (NPs), showing bright red emission, complete ON-OFF contrast with full reversibility, and excellent fatigue resistance. Most interestingly, upon successive UV and visible light irradiation, the NPs exhibit a complete fluorescence quenching and recovery at very low photochromic conversion levels (<5 %), leading to the fluorescence photoswitching of 420±20 molecules for only one converted photochromic molecule. This "giant amplification of fluorescence photoswitching" originates from efficient intermolecular energy-transfer processes within the NPs.
To demonstrate light-path manipulation in arbitrary shapes we fabricated coupled-resonator optical waveguides (CROWs) having a 90 degrees-corner structure on a lithographically patterned substrate. The spectra of propagation light within the CROWs were directly measured by guide-collection-mode near-field scanning optical microscopy. The spectra revealed that the propagation light through the CROWs has a larger transverse-magnetic polarization mode than a transverse-electric (TE) one. The most plausible cause of the lower intensity in the TE mode is that light leaks out to the Si substrate.
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